Optical waveguide signal amplifiers comprise a glass, optical waveguide fiber doped with an optically active material. The dopant, for example, is a rare earth metal oxide, and an amplifier effect is developed by stimulated emission. Pump light is injected into one end of the amplifier fiber. The pump light is emitted from a laser at the wavelength of absorption of the doped glass amplifier fiber. Modulated signal light is also injected at a wavelength in the spectral band of emission of the doped glass amplifier. An amplified optical signal is extracted from the waveguide at the other end of the amplifier fiber.
Such an amplifier is described in E. Desurvire et al., "High-Gain Erbium-Doped Traveling-Wave Fiber Amplifier", Optical Letters, Vol. 12, No. 11, pages 388-390, November, 1987. An amplifier of this type is also described by M. C. Farries et al., "Operation of Erbium-Doped Fiber Amplifiers and Lasers Pumped with Frequency-Doubled Nd:YAGL Lasers", Journal of Lightwave Technology, Vol. 7, No. 10, pages 1474-1477, October, 1989.
Such amplifiers are intended for use in long distance, optical fiber, telecommunications systems which today appear very promising. They present the advantage of eliminating the opto-electronic conversion of signals by acting directly on the optical signal. In order to achieve the level of amplification necessary, an amplifier waveguide is currently on the order of one to several meters in length. This is for a signal transmitted at a wavelength of 1.5 microns, one of the typical wavelengths used in optical telecommunications.
Ion-exchanged materials doped with rare earth metals have been described. See, for example, Najafi et al., "Ion-Exchanged Rare-Earth Doped Waveguides", SPIE, Vol. 1128, Glasses for Opto Electronics, 1989, pages 142-144. An example described in that publication is a slab waveguide made by Ag.sup.+ -Li.sup.+ ion exchange in a neodymium-doped, lithium silicate glass substrate.
U.S. Pat. No. 5,128,801 (Jansen et al.) describes an optical signal amplifier with a waveguide path integrated into a glass body. The glass body is doped with optically active material. The signal to be amplified is transmitted through the waveguide, and the pump power is coupled into the waveguide at one end.
The present invention is concerned with a family of glasses that find particular utility in production of such a signal amplifier. These glasses are capable of being doped with high contents (up to 5 wt. % ) of erbium oxide. Thus, they are characterized by long, excited-state lifetimes.
It is known that boron has a strong competitive effect on the excited-state, radiative transition to the ground state in erbium-doped glasses. This effect occurs through the multiphonon, non-radiative relaxation that leads to very low, excited state lifetimes. Accordingly, glasses of the present invention are boron-free. At the same time, the glasses possess the physical and chemical properties required for ion-exchange.